Manhole structures are arranged at predetermined intervals along sewage lines, for example, for inspection and cleaning purposes, and for providing an intersection between two branches of a sewer line merging into a third, to name just a few applications.
A manhole structure is typically comprised of a manhole base, an intermediate section and a top riser section. The manhole base is placed beneath grade at a depth dependent upon the depth of the sewer line entering the base, through an opening made water-tight by a cooperating gasket. The intermediate and riser sections are utilized to bring the manhole structure up to grade, and provide a water-free, hollow interior which may be easily entered for repair and/or maintenance. The open top of the manhole structure is sealed with a manhole cover removably supported upon a supporting frame, said frame resting either directly upon the top of the riser section or upon the top surface of a grade adjustment ring which may, for example, be one of a plurality of grade adjustment rings positioned between the top surface of the riser section and the bottom surface of the manhole cover support frame, in order to bring the frame opening supporting the manhole cover up to grade.
In order to prevent subsurface water from entering into the interior of the manhole structure through the region of the engaging surfaces between the manhole cover and the adjustment rings, and/or between the engaging surfaces of adjacent adjustment rings, and/or between the engaging surface of an adjustment ring and the top surface of the riser section, mortar is placed between these engaging surfaces. However, due to irregularities in the engaging surfaces; shrinkage and dynamic loading, natural erosion and deterioration of the mortar due to frost heave, surface water eventually seeps through the above-mentioned regions to enter into the interior of the manhole structure.
It is further desirable to install manhole structures in such a way as to limit on-site activities to a minimum and it is, therefore, desirable, no only to provide a water-tight seal in the region between the manhole riser section and the manhole cover support frame, but to be able to accomplish this through the use of a manhole structure, which is of simplified design and is easy to install and does not require additional, complicated manual activity at the installation site.
One technique for accomplishing the above results is described in copending application Ser. No. 514,584 filed July 18, 1983 which discloses a riser section incorporating an integral plastic sleeve, having a flange cast into the riser section. The height of the sleeve is sufficient to accommodate rather large adjustment gaps between the manhole cover supporting frame and the top of the riser section and is preferably formed of a plastic material which, although having excellent ability to withstand erosion and/or deterioration over a long period of time, is easy to cut, in order to position the trimmed cut edge of the free end of the sleeve immediately beneath the shoulder of the manhole cover support frame.
The method for installing the apparatus of the above-mentioned copending application is as follows:
The manhole base, intermediate and riser sections are installed into the ground using conventional techniques and such that the aforementioned plastic sleeve, whose flange is cast into the top region of riser section extends upwardly and above the top surface of the riser section and has an outer diameter which is slightly less than the inner diameter at the open upper end of the riser section.
Dependent upon the depth of the top surface of the riser section relative to grade, none, one or more than one grade adjustment ring is positioned upon the top surface of the riser section. If desired, mortar is placed in the regions between the engaging faces of said members.
The distance between the top surface of the riser section and the interior shoulder of the manhole cover supporting frame is determined, and the top portion of the free end of the sleeve is removed by a simple cutting operation, to locate the top edge of the sleeve just beneath the aforesaid shoulder of the manhole cover supporting frame, thereby completing the installation operation. Since the sleeve providing the water-tight capability is cast into the riser section at the factory, no positioning and/or securement of the sleeve to the riser section is necessary, thus greatly simplifying installation. The sleeve prevents water from entering into the interior of the manhole structure.
The disadvantage of the plastic sleeve disclosed in the aforementioned copending application Ser. No. 514,584 is that the sleeve is easily damaged or broken during handling and especially during transportation, rendering the sleeve, and hence the riser section, useless.
It is further important to provide manhole assemblies having a manhole cover which is water-tightly sealed, an arrangement that is especially useful in low-lying areas which are frequently subjected to flooding. It is not possible to provide the waterlock arrangement of aforementioned Application Ser. No. 514,584 with a water-tight manhole cover.